The present invention relates to a communication terminal apparatus, base station apparatus, and radio communication method to be used in a cellular communication system.
In a cellular communication system, one base station performs radio communication with a plurality of communication terminals simultaneously, and therefore, as demand has increased in recent years, so has the need for higher transmission efficiency.
One technology that has been proposed for increasing the transmission efficiency of a downlink from a base station to a communication terminal is HDR (High Data Rate). HDR is a communication method whereby a base station performs scheduling for allocating communication resources to communication terminals by time division, and also sets a transmission rate for each communication terminal according to the downlink channel quality.
The operations by which a base station and communication terminals perform radio communication with HDR are described below. First, the base station transmits a pilot signal to each communication terminal. Each communication terminal estimates the downlink channel quality using a CIR (desired carrier to interference ratio) based on the pilot signal, etc., and finds a transmission rate at which communication is possible. Then, based on the transmission rate at which communication is possible, each communication terminal selects a communication mode, which is a combination of packet length, coding method, and modulation method, and transmits a data rate control (hereinafter referred to as xe2x80x9cDRCxe2x80x9d) signal indicating the communication mode to the base station.
The type of modulation method that can be used in each system is predetermined as BPSK, QPSK, 16QAM, 64QAM, and so forth. Also, the type of coding that can be used in each system is predetermined as xc2xd turbo code, ⅓ turbo code, xc2xe turbo code, and so forth. Further, a plurality of transmission rates that can be used in each system are predetermined according to a combination of packet length, modulation method, and coding method. Each communication terminal selects a combination whereby communication can be performed most efficiently with the current downlink channel quality, and transmits a DRC signal indicating the selected communication mode to the base station. Generally, DRC signals are represented by numbers from 1 to N, with a higher number indicating a proportionally better downlink channel quality.
Based on the DRC signal transmitted from each communication terminal, the base station sets a transmission rate for each communication terminal, and sends a signal to each communication terminal via a control channel indicating communication resource allocation to each communication terminal. Generally, taking improvement of system transmission efficiency into consideration, communication resources are allocated with priority to the communication terminal that has the best downlink channel quality-that is to say, the communication terminal that transmits the highest-numbered DRC signal.
The base station then transmits data only to the relevant communication terminal in its allocated time. For example, if time t1 has been allocated to communication terminal A, in time t1 the base station transmits data only to communication terminal A, and does not transmit data to a communication terminal other than communication terminal A.
In this way, data transmission efficiency has conventionally been increased for the overall system by setting a transmission rate for each communication terminal according to channel quality by means of HDR, and performing communication resource allocation with priority to a communication terminal with a high transmission rate at which communication is possible.
However, if the communication mode determined by a communication terminal is received erroneously by the base station due to deterioration of the channel conditions on the uplink from the communication terminal to the base station, or the like, the base station will transmit data using that erroneous mode. As the determined communication mode and the communication mode of data transmitted to the communication terminal are different, the communication terminal cannot demodulate or decode the data.
Also, when a base station such as that described above has allocated time t1 to communication terminal A, in time t1 the base station transmits data only to communication terminal A, and does not transmit data to a communication terminal other than communication terminal A.
Due to the above, a problem arises in that, if the communication mode determined by a communication terminal is received erroneously by the base station, there will be an interval during which time-divided communication resources are not used, and downlink throughput falls.
It is an object of the present invention to provide a communication terminal apparatus, base station apparatus, and radio communication method that make it possible to prevent a fall in downlink throughput in a communication system in which communication resources are allocated to communication terminals based on downlink channel quality.
In order to achieve the above-described object, in the present invention, with respect to information, among information indicative of downlink channel quality, which has a possibility of decreasing the downlink throughput when the information is received erroneously in a base station, a communication terminal provides such information with less susceptibility to errors in the propagation path to transmit. It is thereby possible to prevent the downlink throughput from decreasing.